1. Field of the Invention
The present invention relates to a submarine branching unit (herein after simply called branching unit) which branches a transmission line in two different directions in a communication system using the submarine cables, or more particularly to an improvement of the structure of the branching unit.
In order to further accelerate the development of information communication including the Internet via such submarine cables, it is in demand to shorten the time required to deliver communication equipment to customers after a contract has been made. In fact, submarine equipment such as a repeater has recently come to be manufactured within about half the period required several years ago To install or lay a repeater and a submarine cable underseas, a dedicated ship is used and therefore, development of the submarine equipment which is easy to install and lay in the sea is in demand.
There is a similar demand for a branching unit. Especially, a small-sized and light-weight branching unit which is easy to lay in and draw out of the sea and also easy to transport and store. Also, the demand arises since a dedicated strong cable is used with the branching unit and since a sheave having large a diameter as 3 meters is used with the cable.
2. Description of the Related Art
FIG. 1 shows an external appearance of a conventional branching unit with cable terminations. FIG. 2 is a sectional side view of the main body of the branching unit as shown in FIG. 1.
The branching unit 1 is composed of an inner unit 3, cable containers 4, a submarine cable 5, cable terminations 6 and gimbal joints 7. The inner unit 3 is accommodated in a pressure-tight cylinder 2. The cable containers 4 located at both ends of the pressure-tight cylinder 2, contain pigtail cables including an optical fiber for signal transmission and a feeder line for power supply. The cable terminations 6 connect the submarine cable 5 to the cables within the branching unit. The gimbal joint 7 bendably couples the cable containers 4 to the cable terminations 6.
As shown in the FIG. 3xe2x80x94side view, FIG. 4xe2x80x94top plan view and FIG. 5xe2x80x94bottom plan view, the inner unit 3 has vacuum relays 8 and circuit units 9-1 to 9-4 arranged respectively on top and bottom sides of the circuit board. The terminals (not shown in the figure) of the vacuum relays 8 project to the side of the vacuum relays 8. Also, feeder lines (not shown in the figure) connecting to the vacuum relay terminals and the respective circuits project in parallel with the vacuum relay terminals. Therefore, a space is required that is large enough to withstand 15 kilovolts across the terminals of adjacent vacuum relays 8 and accordingly, the vacuum relays 8 are mounted separately on both (upper and lower) sides of the circuit board. Four circuit units 9-1 to 9-4 are each mounted in a box case such that they are superposed on each other. Thus, the inner unit 3 is finished to an outside diameter (D) of 300 millimeters and a length (L) of 550 millimeters.
With the main body of the thus-constructed branching unit as shown in FIG. 2, the pressure-tight cylinder 2 has an outside diameter D1 of about 400 millimeters. The cable container 4 has an outside diameter D2 of about 480 millimeters. The main body has a length L1 of about 1600 millimeters and a weight of about 8 kilonewtons. The total length L2 of the branching unit as measured between the cable terminations 6 on both sides (see FIG. 1) is about 4500 millimeters.
With an ever increasing demand for the communication via the submarine cables, more and more communication capacity is required and accordingly, larger electric power and broader bandwidth are required. As a result, power consumption increases to about 150 watts, i.e., three times as large as the current 50 watts and it is required to increase the withstand voltage characteristics from the current 15 kilovolts to 18 kilovolts.
Accordingly, it is necessary to design a branching unit having improved withstand voltage and heat radiation characteristics, while minimizing the size of the unit. The conventional branching unit had following structural problems when laying/drawing out the unit in/from the sea.
First, since the branching unit is large in size, the unit does not wind well around a sheave whose diameter is about 3 meters and the laying/drawing work had to use a crane and a special rope and therefore, a such-equipped ship or work barge was necessary for that work.
The branching unit, which is about twice as heavy as a repeater, required a submarine cable which can tolerate a high tension applied to the unit in the deep sea. Therefore, a special cable had to be manufactured, taking into consideration the maintenance work after laid in the sea.
An object of the present invention is to provide a branching unit which has excellent withstand voltage and heat radiation characteristics and which is the same in size as a submarine repeater.
Another object of the present invention is to provide a branching unit which has the weight, the distance between gimbal joints on both sides and the outside diameter of the pressure-tight cylinder, reduced to those of a submarine repeater.
Throughout the above-mentioned drawings, identical reference numerals are used to designate the same or similar component parts.
To achieve the above and other objects, the present invention provides a branching unit including in a pressure-tight cylinder, a circuit board, at least one circuit unit mounted on the circuit board and a plurality of vacuum relays mounted on the circuit board and each having respective terminals and wirings. The circuit unit is mounted on one side of the circuit board. The vacuum relays are mounted on the other side of the circuit board. The vacuum relays are each accommodated in the insulator covers along with the respective terminals and wirings. Further, the present invention provides a branching unit including a pair of cables for carrying a signal and power, which cables are each spiraled and are engaged with each other in an interleaved and overlapped fashion on the outside of a cover of the pressure-tight cylinder.